Xenical (orlistat) is a new anti-obesity drug which limits fat absorption by the inhibition of pancreatic lipase (PL) activity. Xenical may inhibit systemic plasma lipases as well as gastrointestinal lipases. The effect or lack thereof that Xenical or its metabolites have on lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activity during the time triglycerides appear in the circulation after eating needs to be established. Therefore, the effects of Xenical and its metabolites on postprandial pre-and postheparin LPL and HTGL activities and on postprandial plasma lipoprotein concentrations were examined in this clinical trial.